1. Field of the Invention
This invention relates to a sensor of the enzyme electrode type comprising an improved membrane and to an analytical method using the sensor.
2. Description of the Related Art
Enzyme electrodes are increasingly used in medical and other laboratories particularly for the determination of materials such as glucose and urea in specimens of blood and other physiological fluids. Such electrodes are described in many publications notably an article by Clark and Lyons (Annals of the New York Academy of Science, 102, 29-45, 1962) and U.S. Pat. Nos. 3,539,455 and 3,979,274 to Clark and Newman respectively. Enzyme electrodes are generally used to determine materials which themselves are not electrochemically active but which in the presence of suitable enzymes take part in reactions which produce species which can be readily detected by the electrodes. In enzyme electrodes the enzymes are frequently located within polymeric materials in close proximity to the underlying electrode.
A considerable amount of research has been carried out in order to improve the properties of membranes for use in enzyme electrodes and many membranes for this purpose have been disclosed. An example of a type of membrane which is often used is the laminated membrane disclosed by Newman in U.S. Pat. No. 3,979,274. This membrane comprises a first or inner layer of an essentially homogeneous material, for example cellulose acetate, which can prevent the passage of materials of low molecular weight likely to interfere with the enzymic signal, a close adherent layer of the enzyme itself (with or without such other materials that may be blended with it), and a second layer (in this instance an outer layer) of a porous support film which can prevent the passage of cellular and colloidal elements.
The determination of glucose can be taken as an example of the determination of a material by an enzyme electrode. In the presence of the enzyme glucose oxidase the following reaction occurs: ##STR1## The hydrogen peroxide produced in this reaction passes through the first layer of a membrane such as that of U.S. Pat. No. 3,979,274 and can be determined using the electrode. Since the hydrogen peroxide produced is dependent upon the glucose present in a specimen, the glucose concentration can be determined using a suitably calibrated sensor.
To date a number of difficulties have limited the utility of enzyme electrodes and restricted the scale of their use in routine analysis of, e.g. blood samples. Significant among these difficulties is the limited linearity of the response of electrodes to analytes such as glucose or lactate which are substrates for the enzyme catalysed reactions. The response is linear only over a limited range of low concentrations of the analytes and hence the concentrations of the materials to be determined must be low and generally diluted samples must be used in specimens for analysis using enzyme electrodes. It is not always practicable to make diluted samples for routine analysis outside the laboratory and it would be impossible for invasive monitoring.